Project Summary/Abstract The identification of environmental factors such as commensal bacterial metabolites that modulate immune responses could provide therapeutic targets for immune-driven diseases. Indeed T helper 17 (Th17) cells, a major immune cell type involved in autoimmunity, are regulated by the microbiota. Our long-term goal is to identify immune-modulatory members of the microbiota and understand the mechanisms whereby they influence the immune system in diverse environmental contexts. Here, we propose a distinct mechanism of microbial-driven Th17 induction where resident gut bacteria produce metabolites that traverse the gut barrier resulting in Th17 activation. Moreover, we hypothesize that this immune manipulation is altered by the diet. These hypotheses stem from our preliminary studies on the prevalent gut bacteria, Eggerthella lenta, whose levels are increased in patients with the Th17 driven disease, rheumatoid arthritis (RA). We found that E. lenta colonization induces Th17 cells and contributes to more severe colitis in a gnotobiotic mouse model. Critically, high arginine conditions lead to decreased activity of an E. lenta-metabolizing enzyme, Cgr2, and our initial studies indicate that high Arg decreases E. lenta-mediated Th17 induction in a Cgr2 dependent manner. Based on these preliminary findings we propose to 1) Identify the mechanism by which E. lenta induces Th17 cells 2) Determine the influence of E. lenta colonization on mouse models of RA and inflammatory bowel disease (IBD) and 3) Assess the effect of dietary Arg on E. lenta?s induction of Th17 cells and autoimmunity. By manipulating Cgr2 though heterologous expression and natural E. lenta strain variants that possess or lack Cgr2, we will investigate the role of this bacterial protein in Th17 induction. To identify the bacterial product produced by E. lenta responsible for Th17 induction we will perform mass spectrometry on HPLC fractionated E. lenta conditioned media, which we have shown to have Th17 inducing capabilities compared to a media control. Further, we will examine if E. lenta?s induction of autoimmunity is altered by dietary conditions using mouse models of RA and IBD and high and low Arg diets. These studies will provide insight into unique mechanisms of Th17 induction and examine how dietary alteration and bacterial metabolites affect autoimmunity. Our novel approach of investigating immune regulation with a microbial-centric perspective will aid in uncovering new mechanisms of immune regulation where the dietary alterations of immune-active microbial metabolites could provide effective therapy. These proposed studies will provide a rich training experience where our multidisciplinary approach will supply exceptional opportunities to expand skills in microbiology, microbiota studies, immunology, and biochemistry. We believe the outstanding research climate at UCSF of collaboration and expertise in these fields will steer this project and training to success.